1. Field of the Invention
This invention relates to a reducing-atmosphere-resistant thermistor element capable of accurately detecting a temperature in a broad temperature range and having stable characteristics even in a reducing atmosphere, and a production method of such a thermistor element. The present invention is particularly suitable for use in temperature sensors for automobile exhaust gases.
2. Description of the Related Art
Thermistor elements for temperature sensors have been used for measuring temperatures in medium and high temperature ranges of about 400 to 1,300xc2x0 C. such as the temperatures of automobile exhaust gases, the gas flame temperatures of gas hot water supply systems, the temperatures of heating furnaces, and so forth. The characteristics of the thermistor elements of this kind are generally represented by a resistance value and a resistance-temperature coefficient (temperature dependence of the resistance value). To provide a practical resistance value to a temperature detection circuit constituting the temperature sensor, the resistance value of the thermistor element must remain within a predetermined range (100xcexa9 to 100 kxcexa9 within an ordinary use temperature range, for example). Perovskite type composite oxide materials have been used mainly as the materials having resistance value characteristics suitable for the thermistor element.
Japanese Unexamined Patent Publication (Kokai) No. 6-325907, for example, discloses a thermistor element using peroviskite type materials. To provide a thermistor element that can be used over a broad temperature range, this reference teaches to mix oxides of Y, Sr, Cr, Fe, Ti, etc, in a predetermined composition ratio and to bake the mixture to obtain a thermistor element that is in a completely solid-solution state. The reference describes that this thermistor exhibits stable characteristics in a high temperature range.
In temperature sensors for automobile exhaust gases, a metal case covers a thermistor element, as a sensing element, at the distal end of the temperature sensor to prevent deposition of dust and soot from the exhaust gases. When the exhaust gas reaches a high temperature of around 900xc2x0 C., however, the metal case is thermally oxidized by the heat of the high temperature exhaust gas, and the internal atmosphere of the metal case is likely to change to a reducing atmosphere. In consequence, a problem develops in that oxides constituting the thermistor element are reduced and the resistance value changes.
To cope with this problem, the temperature sensor is generally placed into an electric furnace and is subjected to thermal aging treatment at 900 to 1,000xc2x0 C. for about 100 hours to stabilize the resistance value. However, if any hole appears in the metal case or the exhaust gas enters the case due to loosening of the case, during use, of the temperature and the thermistor element is exposed to the reducing atmosphere, the resistance value is likely to change as described above. In recent engine control systems, the temperature sensor is often mounted at a position closer to the engine that generates the high-temperature exhaust gas. Therefore, an exhaust gas having a high temperature (e.g. 1,100 to 1,200xc2x0 C.) reaches the temperature sensor. During the thermal aging treatment at 900 to 1,000xc2x0 C., there is the possibility that the metal case is re-oxidized depending on the operation mode of the engine, the thermistor element is again reduced, and the resistance value changes. In other words, a thermal aging treatment cannot completely solve the problem. In addition, the number of production steps increases and invites an increase in the cost of production of in temperature sensor.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 9-69417 describes a technology that shapes a metal case by machining a specific metal material such as an alloy consisting of Nixe2x80x94Crxe2x80x94Fe as a principal component, suppresses the change of the atmosphere inside the case and minimizes the resistance value change of a thermistor element. However, when the metal case is made of the specific metal material, the material cost and the machining cost increase. Further, the problem of the resistance change when the thermistor element itself is exposed to the reducing atmosphere remains to be solved.
As described above, a thermistor element that exhibits stable resistance value characteristics even under the condition where the internal atmosphere of the metal case of the temperature sensor becomes a reducing atmosphere is not available.
In view of the problems described above, the present invention aims at economically providing a reduction-resistant thermistor element the resistance value of which does not greatly change even when the thermistor element is exposed to a reducing atmosphere, and which has high accuracy and excellent resistance value stability.
To solve the problems, the inventors of the present invention have conducted intensive studies and have discovered that when an oxygen occlusion-release composition is added to, and dispersed in, a composition constituting a thermistor element so as to provide the thermistor element with a function of occluding and releasing oxygen in accordance with an environment, migration of oxygen from the thermistor element can be limited and the resistance change can be suppressed in the reducing atmosphere.
The invention according to claim 1 is completed on basis of the finding described above. The thermistor element of this invention is characterized by a construction which consists of a metal oxide sintered body as a principal component, and in which an oxygen occlusion-release composition having oxygen occlusion-release characteristics is dispersed in the metal oxide sintered body.
As the thermistor element according to the present invention has a construction in which the oxygen occlusion-release composition is dispersed in the metal oxide sintered body having thermistor characteristics. Therefore, when the internal atmosphere of the metal case becomes a reducing atmosphere, the oxygen occlusion-release composition emits absorbed oxygen and prevents migration of oxygen from the metal oxide sintered body. In consequence, the thermistor element can prevent fluctuation of the composition resulting from the reduction of the metal oxide sintered body and the resistance change resulting from the former, and can improve resistance value stability.
As described above, the thermistor element according to the present invention has reducing-atmosphere-resistance and its resistance value does not greatly change even when it is exposed to the reducing atmosphere. Therefore, the thermistor element can accurately detect the temperature for a long time, and can provide a temperature sensor having high reliability. Since the metal case need not be made of a specific metal material and a thermal aging treatment is not necessary, either, the production cost can be reduced.
According to a second aspect of the present invention, there is provided a reducing-atmosphere-resistant thermistor element wherein the oxygen occlusion-release composition is an oxide containing at least one metal oxide selected from the group consisting of Ce, Pr and Tb.
According to a third aspect of the present invention, there is provided a occlusion-release thermistor element, as described above, wherein the oxygen occlusion-release composition is at least one oxide selected from the group consisting of CeO2, Pr6O11, Tb4O7, 2CeO2.Y2O3 and CeO2.ZrO2. Since these oxides have the function of occluding and releasing oxygen in accordance with the environment, the reducing-atmosphere-resistance of the thermistor element can be drastically improved when these oxides are added and dispersed.
According to a fourth aspect of the present invention, there is provided a reducing-atmosphere-resistant thermistor element, wherein a starting material of the oxygen occlusion-release composition is ultra-fine particles having a mean particle diameter of not greater than 100 nm. When the ultra-fine particles are used, the particles can be dispersed more uniformly into the metal oxide sintered body constituting the thermistor element, and reducing-atmosphere-resistance can be highly improved.
According to a fifth aspect of the present invention, there is provided a reducing-atmosphere-resistant thermistor element, wherein an addition amount of the oxygen occlusion-release composition is 1 to 95 mol % on the basis of the total molar amount (100%) of the metal oxide sintered body and the oxygen occlusion-release composition. The reducing-atmosphere-resistance improving effect due to occlusion and release of oxygen can be effectively obtained within this range.
According to a sixth aspect of the present invention, there is provided a reducing-atmosphere-resistant thermistor element, wherein the metal oxide sintered body has negative thermistor characteristics. The present invention preferably uses a thermistor element having linear negative characteristics (the resistance value decreases with the increase of the temperature) with respect to the absolute temperature. Such a thermistor element is useful as a temperature sensor. The effect of the present invention is high particularly in vehicles, etc, because the thermistor element is likely to be exposed to the reducing atmosphere.
According to a seventh aspect of the present invention, there is provided a reducing-atmosphere-resistant thermistor element wherein the metal oxide sintered body is a mixed sintered body (M1 M2)O3.AOx of a composite oxide expressed by (M1 M2)O3 and a metal oxide expressed by AOx. M1 in the composite oxide (M1 M2)O3 is at least one kind of element selected from the group consisting of the Group 2A and the Group 3A with the exception of La of the Periodic Table, M2 is at least one kind of element selected from the group consisting of the Groups 3B, 4A, 5A, 6A, 7A and 8, the metal oxide AOx has a melting point of not lower than 1,400xc2x0 C., and a resistance value (at 1,000xc2x0 C.) of the AOx single substance is at least 1,000xcexa9 in a thermistor form.
For temperature sensors used in a wide temperature range, it is advisable to use a mixed sintered body of a composite oxide (M1 M2)O3 of a perovskite structure having relatively low resistance value characteristics within a temperature range of from room temperature to 1,000xc2x0 C., and a metal oxide AOx having a high resistance value and a high melting point. Since the metal oxide AOx has a high resistance value, it can increase the resistance value of the mixed sintered body in the high temperature range. Since it has a high melting point and is excellent in heat resistance, it can improve the high temperature stability of the thermistor element. In this way, it is possible to obtain a wide-range type thermistor element the resistance value of which is within a range of 100xcexa9 to 100 kxcexa9 in a temperature range of from room temperature to 1,000xc2x0 C., and which has a small change of the resistance value due to heat history, etc, and is excellent in stability.
According to an eighth aspect of the present invention, there is provided a reducing-atmosphere-resistant thermistor element wherein, when a molar fraction of the composite oxide (M1 M2)O3 in the mixed sintered body is a and a molar fraction of the metal oxide AOx is b, a and b satisfy the relation 0.05xe2x89xa6a less than 1.0, 0 less than bxe2x89xa60.95 and a+b=1. When the molar fractions a and b satisfy these relation, the effects described above (the resistance value within the predetermined range and resistance value stability) can be accomplished more reliably.
Because the molar fractions can be varied in a broad range as described above, the resistance value and the resistance-temperature coefficient can be variously controlled with a broad range by appropriately mixing and baking (M1 M2)O3 and AOx.
According to a ninth aspect of the present invention, there is provided a reducing-atmosphere-resistant thermistor element wherein M1 in the composite oxide (M1 M2)O3 is at least one kind of element selected from the group Mg, Ca, Sr, Ba, Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Yb and Sc, and M2 is at least one kind of element selected from the group consisting of Al, Ga, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt.
According to an eleventh embodiment of the present invention, at least one metal oxide is selected as the concrete metal oxide AOx from the group consisting of MgO, Al2O3, SiO2, Sc2O3, TiO2, Cr2O3, MnO, Mn2O3, Fe2O3, Fe3O4, NiO, ZnO, Ga2O3, Y2O3, ZrO2, Nb2O5, SnO2, CeO2, Pr2O3, Nd2O3, Sm2O3, Eu2O, Gd2O3,Tb2O3, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, HfO2, Ta2O5, 2MgO.2SiO2, MgSiO3, MgCr2O4, MgAl2O4, CaSiO3, YAlO3, Y3Al5O12, Y2SiO5 and 3Al2O3.2SiO2. All these metal oxides have a high resistance value and high heat resistance and contribute to an improvement in the performance of the thermistor element.
According to a twelfth aspect of the present invention, M1 in the composite oxide (M1 M2)O3 is Y, M2 is Cr and Mn, A in the metal oxide AOx is Y, and the mixed sintered body (M1 M2)O3.AOx is Y(CrMn)O3.Y2O3. This mixed sintered body is suitably used for temperature sensors, etc, and exhibits high performance in a wide temperature range.
According to a thirteenth aspect of the present invention, a sintering assistant is added to the mixed sintered body (M1 M2)O3.AOx to improve sintering property of each particle. At least one of CaO, CaCO3, SiO2 and CaSiO3 is used as a sintering assistant, and a thermistor element having a high sintering density can be obtained.
According to a fourteenth aspect of the present invention, there is provided a method of producing a reducing-atmosphere-resistant thermistor element consisting of a metal oxide sintered body as a principal component, comprising the steps of mixing and pulverizing a raw material powder containing the metal, heat treating the mixture and obtaining a thermistor composition made of an oxide containing the metal; mixing and pulverizing the thermistor composition and an oxygen occlusion-release composition having oxygen occlusion-release characteristics to prepare a composite raw material containing the oxygen occlusion-release composition dispersed in the thermistor composition; and molding the composite raw material into a predetermined shape and baking the resulting molded article.
As a result of studies of production processes for improving the reducing-atmosphere resistance of the thermistor element, the present inventors have found that it is important to add and uniformly disperse the oxygen occlusion-release composition into the metal oxide sintered body constituting the thermistor element. Therefore, after the thermistor composition that is to function as the raw material of the metal oxide sintered body is prepared, the oxygen occlusion-release composition is mixed and pulverized, and the thermistor element is obtained through molding and baking. In this way, there is provided a reducing-atmosphere-resistant thermistor element that has resistance stability even in a reducing atmosphere.
According to a fifteenth aspect of the present invention, the raw material powder used in the fourteenth aspect is powder having a mean particle diameter of not greater than 100 nanometers. The composition prepared by mixing, pulverizing and heat treating raw material powder of ultra-fine particles having a mean particle diameter of not greater than 100 nanometers has small variance of the composition, reduces variance of the resistance of the thermistor element and further improves the temperature accuracy.
According to a sixteenth aspect of the present invention, there is provided another method, of producing a reducing-atmosphere thermistor element, comprising the steps of mixing a precursor compound of the metal oxide into a liquid phase to prepare a precursor solution, heat treating the precursor solution and obtaining a thermistor composition containing the metal oxide; mixing and pulverizing the thermistor composition and an oxygen occlusion-release composition having oxygen occlusion-release characteristics to prepare a composite raw material containing the oxygen occlusion-release composition dispersed in the thermistor composition; and molding the composite raw material into a predetermined shape and baking the resulting molded article.
When the thermistor composition as the principal component of the thermistor element is prepared, the present invention uses a solution containing a precursor compound of the metal oxide described above and heat-treats the solution to obtain the thermistor composition. In this way, the present invention can provide a thermistor composition having a reduced variance in resistance and an improved temperature accuracy. The oxygen occlusion-release composition is mixed and pulverized into the resulting thermistor composition in the same way as described above, and the thermistor element containing the oxygen occlusion-release composition that is uniformly dispersed provides a similar effect in the same way as described above.
According to a seventeenth aspect of the present invention, there is provided another method, of producing a reducing-atmosphere-resistant thermistor element, comprising the steps of mixing a precursor compound of the metal oxide into a liquid phase and preparing a first precursor solution; mixing a precursor compound of an oxygen occlusion-release composition having oxygen occlusion-release characteristics into a liquid phase and preparing a second precursor solution; mixing the first and second precursor solutions and preparing a mixed precursor solution of the metal oxide and the oxygen occlusion-release composition; heat treating the mixed precursor solution and preparing a composite raw material containing the oxygen occlusion-release composition dispersed in the thermistor composition containing the metal oxide; and molding the composite raw material into a predetermined shape and baking the resulting molded article.
When the composite raw material described above is prepared, it is also possible to employ a method that separately prepares the precursor solutions of the metal oxide and the oxygen occlusion-release composition, mixes these solutions to obtain a mixed precursor solution and heat treats the mixed precursor solution. When mixing is done in the solution form, uniform mixing can be made more easily, and a thermistor element in which the oxygen occlusion-release composition is uniformly dispersed can be obtained.
According to an eighteenth aspect of the present invention, there is provided another method, of producing a reducing-atmosphere-resistant thermistor element, comprising the steps of mixing a precursor compound of the metal oxide described above into a liquid phase to prepare a precursor solution, mixing an oxygen occlusion-release composition having oxygen occlusion-release characteristics into the precursor solution and preparing a mixed precursor solution containing the oxygen occlusion-release composition dispersed therein; heat treating the mixed precursor solution and preparing a composite raw material containing the oxygen occlusion-release composition dispersed in the thermistor composition containing the metal oxide; and molding the composite raw material into a predetermined shape and baking the resulting molded article.
When the thermistor composition as the principal component of the thermistor element is prepared, the present invention uses the solution containing the precursor compound of the metal oxide and adds the oxygen occlusion-release composition. In this way, the present invention can mix them easily, and can obtain the thermistor element having the oxygen occlusion-release composition uniformly dispersed therein.
According to a nineteenth aspect of the present invention, there is provided another method, of producing a reducing-atmosphere-resistant thermistor element, comprising the steps of obtaining a mixture by mixing and pulverizing raw material powder containing the metal described above; causing the mixture to be impregnated with a precursor solution of an oxygen occlusion-release composition having oxygen occlusion-release characteristics; heat treating the mixture impregnated with the precursor solution of the oxygen occlusion-release composition and preparing a composite raw material containing the oxygen occlusion-release composition dispersed in the thermistor composition containing the metal oxide; and molding the composite raw material into a predetermined shape and baking the resulting molded article.
When the composite raw material described above is prepared, it is also possible to employ a method that prepares a precursor solution of the oxygen occlusion-release composition described above, causes the raw material mixture of the metal oxide to be impregnated with the precursor solution, and heat treats the solution. Since mixing is conducted in the solution form, uniform mixing can be done easily, and the thermistor element containing the oxygen occlusion-release composition uniformly dispersed therein can be obtained.
According to a twentieth aspect of the present invention, the oxygen occlusion release composition or its starting material is ultra-fine particles having a mean particle diameter of not greater than 100 nanometers. When the ultra-fine particles are used, the oxygen occlusion-release composition can be uniformly dispersed, and the resistance stability of the thermistor element can be improved.
According to a twenty-first aspect of the present invention, there is provided a temperature sensor comprising the reducing-atmosphere-resistant thermistor element according to the first or thirteenth aspect of the present invention. The thermistor element having the construction of any of the aspects described above can detect the temperature over a broad range and has stable resistance value characteristics. Therefore, it is possible to provide a temperature sensor having high performance and excellent reduction resistance.